Deep vs. REM Sleep

While you sleep, your brain cycles through various sleep stages, activating different brainwaves and changing your heart rate and breathing. The most important sleep stages are NREM stage 3 (deep sleep) and REM sleep.

REM is typically the best-known sleep stage, in part thanks to the American rock band REM but mostly because it makes us dream. Every dream (and nightmare) is during REM sleep, making it the most interesting sleep stage.

However, deep sleep is inarguably the most critical sleep stage for health. Deep sleep is when your brain waves slow down, and your body releases growth hormones to build, repair, and grow muscles, bones, and tissue (cells).

In a nutshell – REM sleep is about the brain with increased brain activity, while deep sleep is about the body with reduced brain activity.

You cannot survive without one or the other – you need both sleep stages for your brain and body to continue functioning.

What is deep sleep?

Deep sleep (also known as non-rapid eye movement stage 3 sleep) triggers a brainwave slowdown with enhanced metabolic (bodily) activity. It is very much the case that your brain is sacrificed so your body can maintain itself.

During deep sleep, brainwaves slow down, your pituitary gland releases human growth hormone, and your immune system and metabolism kick into overdrive. Simply put, all the things that make you feel fit are handled.

What is REM sleep?

REM sleep (rapid eye movement sleep) is the dream sleep stage, during which your brainwave activity flies off the chart. Your thalamus is active, sending images, sounds, and even smells (from memories) to fill your dreams.   

Sleep is not as deep during REM due to increased brain activity, but it contributes to memory consolidation and hormone release. A healthy brain needs REM sleep to develop memories and deal with emotional processing.

Which is most important?

Deep sleep is more critical than REM sleep because it is responsible for maintaining more brain/body functions. You feel more rested and healthier with deep sleep, while REM sleep has less of an effect on your health.

The good news is that your body will cycle through deep and REM sleep as needed – it isn't something you can decide. However, being woken from deep or REM sleep will interrupt the optimal flow of your sleep, making you groggy.

How do you know how much you're getting?

Waking up exhausted, morning headaches, excessive daytime sleepiness, and irritability are tell-tale signs that you aren't getting enough deep and REM sleep.

If you go to bed too late or have interrupted sleep (new parents will know this well), you are almost certainly not getting enough sleep.

A fitness band or smartwatch that tracks sleep can help you understand how much time you spend in each sleep stage.

Another option for severe sleep problems is polysomnography (PSG), which tests and monitors your sleep in a laboratory.

What happens when you don't get enough deep REM sleep?

Roughly 75 percent of your sleep is spent in non-REM sleep. Deep sleep accounts for around 23%, while REM sleep accounts for approximately 25%.

So, around half your total sleep time is spread across deep and REM sleep. Without them, a few things will happen to your brain and body:

• Shrinkage in the frontal cortex (this can make it harder to communicate)
• Glymphatic system disruption (this creates a build-up of brain toxins)
• Neural and brain cell disruption (this makes it harder to store memories)
• Metabolic hormone disruption (this increases fat gain)
• Immune disruption (making you more susceptible to sickness and disease).

However, the most apparent symptoms are grogginess, irritability, a short temper, and low concentration. A lack of sleep makes you worse at most things, and a chronic lack of sleep can have serious health consequences.

You need between seven and nine hours of sleep per night to feel healthy, which means a good amount of deep and REM sleep.